Portable non-contact moisture meter including electrodes driven 180 out of phase



w. D. PERRY 3 i 8381 vPORTABLE NONCONTACT MOISTURE METER INCLUDING ELECTRODES June 3, 1969 .DRIVEN 180 OUT OF PHASE Filed July 5, 1966 N VEAVTOR.

D. PERRY Y :M4

wu L /AM ATTORNEYS K United States Patent O U.S. Cl. 324-61 2 Claims ABSTRACT F THE DISCLOSURE A portable detector for measuring the moisture content of wood of nonuniform thickness, which detector measures the internal resistance of the wood and relates it to the moisture content of the Wood through which the electrical impulse hows. The detector comprises a pair of electrodes placed side by side, an oscillator which drives the electrodes 180 out of phase, and an electrical bridge circuit which determines the internal resistance of the wood sample when the wood is placed adjacent the electrodes.

This application is a continuation-in-part of my copending application Ser. No. 319,191, tiled Oct. 28, 1963, now abandoned.

This invention relates to a moisture detector for wood. Specilically, this invention relates to a portable moisture detector that Arapidly measures the water concentration in wood by employing .a pair of electrodes that are not in physical Contact with the wood.

Heretofore, the prior art has suggested many types 0f moisture detector meters. One type is a resistance meter which consists of probes driven into the wood and a suitable readout electric circuit. These meters are slow, not suitable for high speed scanning and leave holes in the wood.

Another type is the power loss meters that consist of electrodes which contact the wood surface setting up an electrical field in the food and a suitable readout electrical circuit therefor. These meters are sensitive to the degree of contact with the wood surface and to the wood thickness. Accordingly, a greater thickness in the wood results in decreased accuracy.

Capacitance type moisture meters have been developed that consist of two electrodes either in contact with or at a iixed distance to the wood. These meters are supposed to measure the increase in capacitance due to the presence of a dielectric other than air adjacent to the electrodes. Generally speaking, -wood has a dielectric constant of about 2, water 80 and air 1. Consequently, the gauges or meters are dependent on wood density and thickness as -well as moisture content. They are also quite sensitive to the distance between the Wood surface and the electrodes, which means that a Wet pocket in the middle of a thick wood sample may not be detected. Accuracy is in doubt if the electrode-to-wood distance varies as is the case with rough sawn lumber.

An object of this invention is to provide a moisture detector that accurately determines the water content in wood.

Another object of this invention is to provide a portable moisture detector that accurately determines the water content of wood regardless of its distribution within the wood.

Another object of this invention is to provide a -portable moisture detector that is provided with a pair of electrodes that do not physically contact the Iwood and is suitable for accurately determining the Water concentration in wood of nonuniform thickness.

3,448,381 Patented June 3, 1969 These and other advantages -will become manifestly clear to those skilled in the art when taken in conjunction with the detailed description and the drawings wherein:

FIGURE 1 is 4a perspective view of the portable moisture detector illustrating the top portion thereof.

FIGURE 2 is a perspective view of the portable moisture detector illustrating the bottom portion thereof.

FIGURE 3 is a schematic wiring diagram illustrating the manner in which the portable moisture meter measures the water content of wood.

FIGURE 4 is a schematic diagram illustrating the electrical analogue of the wood as seen by the electrodes.

Referring now to the drawings in general, the numeral 1 designates the moisture detector meter. The moisture detector 1 is provided with a housing 2 which is provided with a generally U-shaped channel member 3 having laterally spaced apart legs 4 and 5. The legs 4 and 5 are shown as being integrally connected to the member 3 but it is to be understood that the legs 4 and 5 may be detachably secured to the member 3. End blocks 6 and 7 are provided between the legs 4 and 5 adjacent each end of the housing 2. The end blocks 6 and 7 may be secured to the housing in any conventional manner such as for example by screw 8. The end blocks 6 and 7 are formed of any suitable material such as mica or glass or a substitute therefor.

Adjacent each end block 6 and 7 yare mounted electrodes 9 and 10. The electrodes 9 and 10 are secured to insulators or spacers 11, 12 and 13 which in turn are secured to member 3. The electrodes 9 and 10 are electrically isolated from and shielded lby member 3. The spacer members or insulators 11, 12 and 13 -may be formed of any material such as mica or glass or a substitute therefor.

Intermediate the ends of the member 3 is provided a generally L-shaped housing 15 being provided with a handle member 16. The housing 15 may be secured to the member 3 in any conventional manner. Adjacent the front end of the handle member 16 is provided a plunger 17 which closes a switch to allow current to flow to the electronic circuit 30 that includes an oscillator 33, a transformer 34, an amplifier 31 and capacitors 35, 36, 37, 38, 39, 40 within the housing 15.

Current is supplied to the electronic circuit 30 by batteries 25 that are mounted within a battery housing 23 which is connected to the tmember 3 by bolts or screws 24 extending through flanges 24.

Adjacent the front end of the L-shaped housing 15 is provided an indicator meter 18 provided with a scale 19, a pointer 20 and a zero adjustment button 22.

The amplier 31 is provided lfor indicating unbalance of the impedance bridge formed by capacitors 35, 37, 39 and 40'- It has an adjustment knob 21 to adjust the output senstitivity of the amplifier 31. Other meter means for indicating bridge unbalance would be suitable, however. Extending from the housing 15 through an opening 26 in U-shaped member 3 are lead wires 27 and 28 extending to the electrodes 9 and 10. Another lead also extends through opening 26 and is connected to the housing as indicated at 29 to ground the U-shaped member 3.

Accordingly, it can be seen that the U-shaped member 3 with legs 4 and 5 and end blocks 6 and 7 in effect form a shield for the electrode members 9 and 10. It is to be understood that the electrodes 9 and 10 are not in actual contact with the housing member 2 which can be formed of any suitable material as desired.

The electronic circuit 30 is of any conventional design but is preferably a transistor type so that the housing 15 may be kept conveniently small. Moreover, the oscillator 33 within the electronic circuit 30 is so connected to drive the electrodes 9 and 10, 180 out of phase with respect to each other. Electrode 10 is connected to a capacitive voltage divider formed by capacitors 36 and 38 so that it is maintained at approximately the same voltage as electrode 9 although of opposite instantaneous polarity.

The electrodes 9 and 10 are of a predetermined size. The proper function of the electrodes will depend on the area of the wood under each of them. The span of the electrodes 9 and 10 is greater than the maximum width of lumber to be metered and the spacing between the electrodes 9 and 10 is more than twice the maximum thickness of the lumber to be metered. Accordingly, the size, shape and spacing of the electrodes 9 and 10l can be predetermined for successful operation. Moreover, the size, shape and spacing of the electrodes will dictate how far the distance from the shield member 3 the electrodes 9 and 10 will be placed. The electrode members can be formed of any suitable material such as aluminum, stainless steel, copper or other suitable conducting material.

In use, the electrodes 9 and 10 are connected to the electronic circuit 30 by the leads 27 and 28. Moreover, the plate 3 is maintained at a neutral potential by lead 29, The operator need only grasp the handle 16 and place the indicator meter 1 adjacent the wood 32 to be measured. Then the operator will depress the plunger 17 to activate the electronic circuit 30 from the power of batteries 25.

FIGURE 4 shows an electrical analogue of the wood as seen by electrodes 9 and 10V. CEW9 is the capacitance between electrode 9 and the wood While CEW10 is the similar capacitance between electrode 10 and the wood. RW represents the internal electrical resistance of the wood. By means of the oating reference electrode 10, which is at approximately the same voltage but of opposite polarity to the measuring electrode 9, an apparent zero voltage point at the same potential as shield 3 is induced. Electrically this is equivalent to connecting a conductor from the shield 3 to the apparent zero voltage point although there need not be any actual contact between the wood and the meter in the usual direct current sense. Stated another way, the wood need not be grounded to the meter shield 3.

The bridge composed of capacitors 35, 37, 39 and 40 will be unbalanced by the impedance of CEW9 and approximately 1/2 of RW in series being placed in parallel with C37. At low moisture contents the resistive component 0f this parallel impedance will be very high and bridge unbalance will be small. When the wood moisture is very high the value of RW will be low and the impedance intrduced into the parallel circuit by CEW9 will be controlling. Wood moisture content thus correlates with RW. The meter is most accurate when the impedance ratio ZRW/ZCEW9 is in the range 0.1 to l0. It must be emphasized that the internal resistance of the wood is the electrical property being measured and not dielectric constant.

It should be pointed out that at no time do the electrodes 9 and 10 contact the wood surface. The end blocks 6 and 7 insure that the electrodes 9 and 10 are spaced from the wood 32 to be metered What is claimed is:

1. A detector for indicating the moisture content of wood, comprising: a housing; a pair of spaced apart electrodes in side by side relationship supported by said housing; said electrodes being positioned a predetermined distance apart; electronic circuit means including an oscillator and indicating means; said oscillator being connected to said electrodes for driving them 180 out of phase; and means supported by said housing for spacing said spaced apart electrodes a predetermined distance from said housing and a predetermined distance from the lower end of said housing and means to determine the internal resistance 0f Wood when the wood is placed adjacent said electrodes.

2. A detector for indicating the moisture content in wood in accordance with claim 1 wherein said spacing means for said spaced apart electrodes is formed of a nonconducting material.

References Cited UNITED STATES PATENTS 2,535,026 12/1960 Anderson 324-61 2,782,367 2/1957 Dallas 324-61 3,009,101 ll/l96l Locher 324-61 3,339,137 8/1967 Perry 324-61 RUDOLPH V. ROLINEC, Prima/y Examiner'.

E. E. KUBASIEWICZ, Assistant Examiner. 

